The Deep Space 1 (DS1) spacecraft flew within approximately
2100 km of comet 19P/Borrelly on 2001 Sep. 22.94 UT.
Spacecraft optical navigation (OpNav) images were taken
beginning Aug. 25, but the comet was not unambiguously
detected until Aug. 29, at a range of 0.233 AU. Already at
this point it was evident that the DS1 OpNav observations
were inconsistent with predictions, but as more OpNavs were
processed at ever-diminishing ranges it became clear that
the comet was actually more than 1600 km away from the
original ephemeris derived only from ground-based
observations.

This is the first time that deep-space OpNav images have
been used to update a comet or asteroid ephemeris, and the
introduction of such high precision observations was
critical to the success of the flyby, although it created
new challenges for the comet orbit determination. In
particular, the standard modeling of nongravitational
accelerations proved inadequate to such an extent that the
ground-based data arc was reduced from three apparitions to
less than one revolution in order to fit the OpNavs.
Furthermore, unusually large (for Borrelly)
non-gravitational accelerations, especially in the radial
direction, were indicated. This work points out the need for
better modeling, perhaps through the introduction of
stochastic accelerations.

Even with a much shorter data arc it was clear that
precision ground-based astrometry was consistently biased to
the northeast, or anti-sun direction, due to a fundamental
inconsistency between the ground-based and space-based OpNav
observations. Careful study of both OpNav and ground-based
images of Borrelly indicated an asymmetric coma associated
with a distinct anti-tail of material in the sunward
direction. This asymmetry was not well-modeled by the
astrometric reductions, which were based on a best-fitting
2D gaussian. Recomputed astrometry, referenced to the center
of the brightest pixel, placed the ground-based data in
substantially better agreement with the spacecraft
observations, and significantly improved the final ephemeris
used by DS1, which was accurate to within about 100 km, with
most of the error along the spacecraft line-of-sight.

We postulate that the large outward radial accelerations and
the presence of an anti-tail during multiple apparitions
could be explained if the rotation pole were pointing
roughly sunward during perihelion passage.